This invention provides an electrophoretically switchable reflective image display which retro-reflects incident light rays with respect to three mutually perpendicular axes.
U.S. Pat. No. 6,304,365 (the ""365 patent), which is incorporated herein by reference, describes a reflective (front-lit) image display for viewing images in a preferred direction. The display has parallel, macroscopically planar, structured surface (preferably prismatic), light deflecting and reflecting portions which are longitudinally symmetrical in mutually perpendicular directions, both of which are perpendicular to the preferred viewing direction. A liquid electro-phoretic medium containing a particulate suspension contacts the light reflecting portion. A controller applies an electromagnetic force to selectively electrophoretically move the particles into the evanescent wave region adjacent the light reflecting portion to frustrate total internal reflection (TIR) of light rays at selected points on the light reflecting portion.
The structured surfaces on the light deflecting portion deflect light rays incident in the preferred viewing direction toward the light reflecting portion by imparting to the rays a directional component in the direction of longitudinal symmetry of the light reflecting portion. The structured surfaces on the light reflecting portion totally internally reflect the deflected light rays toward the light deflecting portion at points other than the selected points at which TIR is frustrated. Then, the structured surfaces on the light deflecting portion again deflect the totally internally reflected light rays, cancelling the directional component therefrom, such that the deflected totally internally reflected light rays emerge from the display in a direction substantially parallel to the preferred viewing direction.
The directional characteristic of any light ray can be described in terms of three vectors corresponding to three mutually perpendicular axes. For a light ray to undergo xe2x80x9cfull retro-reflectionxe2x80x9d, all three vectors must undergo a directional inversion. An odd number of reflections from a planar reflector oriented perpendicular to a given axis directionally inverts (i.e. reverses the sign of) the component of the ray""s direction vector for that axis. Full retro-reflection requires an odd number of reflections in each one of the three mutually perpendicular directions.
FIGS. 1A and 2 pictorially illustrate two optical geometries disclosed in the ""365 patent. In FIG. 1A, outward and inward thin sheets 10, 12 are separated by fluidic gap 14. Prisms 16 are formed on the inward surface of outward sheet 10, which has a flat outward surface. Prisms 18 are formed on the inward surface of inward sheet 12, which also has a flat outward surface. Prisms 16, 18 extend longitudinally in mutually perpendicular directions: prisms 16 extending substantially parallel to the X axis, and prisms 18 extending substantially parallel to the Y axis. The preferred Z axis viewing direction is mutually perpendicular to both the X and Y axes. A low refractive index medium (not shown) is maintained in gap 14 to reduce the extent to which light rays entering inward sheet 12 are refracted, thus maintaining a high effective refractive index for inward sheet 12. In the FIG. 2 geometry, sheet 20 is formed with mutually perpendicular, longitudinally extending outward prisms 22 and inward prisms 24 on opposite sides of a single sheet 20; prisms 22 extending substantially parallel to the X axis, and prisms 24 extending substantially parallel to the Y axis. An electrophoresis medium (not shown) is maintained in contact with inward prisms 24.
The FIG. 1A geometry is better suited to use by viewer 32 with light rays which are incident in the Z axis direction 34A perpendicular to the flat outward surface of outward sheet 10. Both the X axis vector component and the Z axis vector component of a light ray incident on the FIG. 1A geometry are directionally inverted, but the incident ray""s Y axis vector component is not inverted. The FIG. 1A geometry inverts the vector components of incident light rays in two of the three mutually perpendicular X, Y and Z directions, namely the X and Z directions; without inverting the vector component in the third (Y axis) direction. FIG. 1B depicts inversion of the X and Z components of light rays incident on sheet 12, viewed in cross-section along the Y axis. FIG. 1C depicts inversion of the Z component, but not the Y component, of a light ray incident on sheet 12, viewed in cross-section along the X axis.
In applications such as variable retro-reflectivity image displays, directional inversion of X, Y and Z components is desirable, rendering the FIG. 1A geometry inadequate for such applications. Highly retro-reflective sheets consisting of glass beads or corner-cube structures are currently in widespread use as retro-reflective signs. In the latter case, reflection is caused by TIR in the cube structures, but it is impractical to modulate TIR in such sheets to produce a variable retro-reflectivity image display. For example, it is impractical to fabricate such sheets using materials of sufficiently high refractive index for TIR to occur when the material contacts a suitable electrophoretic medium.
The FIG. 2 geometry, which similarly inverts the X and Z components without inverting the Y component, is better suited to use by viewer 32 with light rays which are incident in direction 34B inclined at 45xc2x0 to the macroscopic plane of sheet 20. Electrophoretically switchable image displays incorporating FIG. 2 type geometric structures are easily fabricated, work well within a reasonably wide angular range of incident light, and are amenable to achieving full retro-reflection in the X, Y and Z directions in accordance with this invention.
The invention provides a three sheet reflective variable image display for retro-reflecting light with respect to mutually perpendicular X, Y and Z axes. The display has a preferred Z axis viewing direction. The first (outermost) sheet is a light deflecting/recombining transmitter sheet which is longitudinally symmetrical with respect to the X axis. The second (innermost) sheet is an X and Z vector components inverting reflector sheet with light deflecting and light reflecting portions. The light deflecting portion is longitudinally symmetrical with respect to the X axis. The light reflecting portion is longitudinally symmetrical with respect to the Y axis. The X and Z vector components inverter sheet is substantially macroscopically parallel to the light deflecting/recombining transmitter sheet. The third (intermediate) sheet is a Y vector component inverting transmitter sheet having longitudinal symmetry with respect to the X axis. The Y vector component inverter is substantially macroscopically parallel to and positioned between the light deflecting/recombining transmitter sheet and the X and Z vector components inverter sheet. The Y vector component inverter has a plurality of microstructure reflector elements with their surface normal substantially parallel to the Y axis. Each element has a height H. Adjacent pairs of elements are spaced apart by a separation distance D.
Light rays within about 25xc2x0 of perpendicular incidence to the light deflecting/recombining transmitter (hereafter called xe2x80x9capproximately perpendicularxe2x80x9d light rays) are transmitted by the light deflecting/recombining transmitter toward the Y vector component inverter and X and Z vector components inverter at an angle xcex8 of about 30xc2x0 to 60xc2x0 with respect to the Z axis. The rays are reflected by the X and Z vector components inverter toward the Y vector component inverter and light deflecting/recombining transmitter at the same angle xcex8 with respect to the Z axis. The vector components of substantially all of the light rays reflected by the X and Z vector components inverter are directionally inverted with respect to the X and Z axes. The vector components of substantially all of the light rays transmitted by the light deflecting/recombining transmitter and reflected by the X and Z vector components inverter are directionally inverted with respect to the Y axis by making only one reflection at one of the Y vector component inverter""s reflector elements before the rays return to the light deflecting/recombining transmitter. The rays return to the light deflecting/recombining transmitter at the same angle xcex8 with respect to the Z axis at which they were initially transmitted by the light deflecting/recombining transmitter. The returned rays are then transmitted by the light deflecting/recombining transmitter sheet toward the viewer within an angular range of about 25xc2x0 of perpendicular to the light deflecting/recombining transmitter, with the angle of each returned ray substantially 180xc2x0 opposed to that of the corresponding incident light ray. That is, a light ray incident on the light deflecting/recombining transmitter at an angle xcex1 with respect to the Z axis, where xcex1 less than 25xc2x0, returns to the viewer at substantially the same angle xcex1, but travels in the opposite direction to the incident ray.
The reflector elements"" H:D aspect ratio is selected to maximize the fraction of light rays which encounter a reflector element only once during the rays"" return passage between the light deflecting/recombining transmitter and X and Z vector components inverter. The desired Y-axis vector component net directional inversion is not imparted to light rays that do not encounter a reflector element, nor to light rays that encounter a reflector element twice.